Conventionally, an ultrasonic imaging is known as a method for obtaining an internal tomographic image of a subject in which an ultrasonic image is generated by projecting an ultrasonic wave into a subject and detecting an ultrasonic wave reflected from the inside of the subject and a morphological tomographic image of the inside of the subject is obtained. In the mean time, the development of systems for displaying not only a morphological image but also a functional tomographic image has been in progress in recent years in the field of subject examination. As one of such systems, a system which uses photoacoustic spectroscopy is known. In the photoacoustic spectroscopy, light having a predetermined wavelength (e.g., visible light, near infrared light, or intermediate infrared light) is projected onto a subject and a photoacoustic wave which is an elastic wave generated in the subject as a result of absorption of the light energy by a particular substance is detected and the density of the particular substance is quantitatively measured (Japanese Unexamined Patent Publication No. 2010-012295). The particular substance in the subject is, for example, glucose or hemoglobin in the blood, or the like. The technology in which a photoacoustic wave is detected and a photoacoustic image is generated based on the detected signal in the manner described above is called photoacoustic imaging (PAI: Photoacoustic Imaging) or photoacoustic tomography (PAT: Photoacoustic Tomography).
Conventionally, the measurement which uses such photoacoustic effect (photoacoustic measurement) has the following problems. That is, the intensity of the light projected onto the subject is significantly attenuated while propagating inside of the subject by absorption and scattering. In addition, the intensity of photoacoustic wave generated inside of the subject based on the projected light is also attenuated while propagating inside of the subject by absorption and scattering. Consequently, it is difficult to obtain information of a deep portion of the subject by the photoacoustic measurement. In order to solve the problem, it is conceivable to enhance the photoacoustic wave to be generated by increasing the amount of energy of the light projected into a subject.